Meteorological monitoring satellites and communications satellites are usually located in Geostationary Earth Orbit—(GEO) or Low Earth Orbit (LEO). GEO satellites appear to be motionless in the sky, providing the satellite with a continuous view of a given area on the surface of the Earth. Unfortunately, such an orbit can only be obtained by placing the satellite directly above the Earth's equator (0° latitude), with a period equal to the Earth's rotational period, an orbital eccentricity of approximately zero and at an altitude of 35,789 km. While such orbits are useful in many applications, they are very poor at covering higher latitudes (not very useful above 60° latitude for weather and climate monitoring nor above 70° latitude for reliable mobile communications). The optical sensors on a GEO meteorological monitoring satellite, for example, would view higher latitudes at such a poor angle (i.e. a low “elevation angle”) that it could not collect useful data. GEO communications satellite links become unreliable or fail as the elevation angle to the satellite decreases with increasing latitude.
Low Earth Orbit (LEO) satellites are placed in circular orbits at low altitudes (less than 2,000 km) and can provide continuous coverage of the circumpolar region but this requires many satellites as each one is over the region for a relatively small amount of time. One operational example is the Iridium system which uses a constellation of 66 satellites. While practical for relatively low bandwidth communications, it is not cost effective for broadband communications or for weather and climate monitoring which require large and expensive payloads to be placed on each satellite. In view of the cost of building, launching and maintaining each satellite this is a very expensive way of providing continuous satellite coverage of a specific geographic area.
Highly Elliptical Orbits (HEO) such as the Molniya and the classic Tundra orbits can provide better converge of high, latitudes with fewer satellites, but both orbits are problematic.
Highly Elliptical Orbits (HEO) are those in which one of the foci of the orbit is the centre of the Earth. The speed of a satellite in an elliptical orbit is a declining function of the distance from the focus. Arranging the satellite to travel close to the Earth during one part of its orbit (the perigee) will cause it to travel very quickly at that time, while at the other end of the orbit (the apogee), it will travel very slowly. A satellite placed in these orbits spends most of its time over a chosen area of the Earth, a phenomenon known as “apogee dwell”. The satellite moves relatively slowly over the areas of interest, and quickly over areas that are not of interest.
The orbital plane of a HEO is inclined with respect to the Earth's equator. An inclination close to 63.4° is chosen in order to minimize the requirement for the satellite on-board propulsion system to maintain the apogee above the service area.
The Molniya orbit is a HEO with an orbital period of approximately 12 hours. The altitude at the perigee of a Molniya orbit is low (on the order of 500 km above the Earth's surface) and the orbit passes through the Van Allen Belts. The Van Allen Belts are belts of energetic charged particles (plasma) around the Earth, which are held in place by Earth's magnetic field. Solar cells, integrated circuits and sensors are damaged by the radiation levels in these belts, even if they are “hardened” or other safety measures are implemented, for example, turning sensors off when passing through regions of intense radiation. Despite these efforts, satellites which may otherwise have a 15 year expected life will only have about a 5 year life if they have to travel regularly through the inner Van Allen belt of high energy protons (the outer belt of electrons is less problematic). This shortened life of satellites makes Molniya systems very expensive.
The classic Tundra orbit is also a Highly Elliptical Orbit, with the same inclination as Molniya (63.4°). It is also a geosynchronous orbit with an orbital period of one sidereal day (approximately 24 hours). The only operational system in Tundra orbit is Sirius Satellite Radio, which operates a constellation of three satellites in different planes, each satellite plane being offset by 120°, to provide the coverage they desire for their broadcast radio system. Two satellites in a classic Tundra orbit could not provide continuous coverage of a circumpolar region.
Even in view of the problems with the Molniya (short design life) and the classic Tundra systems (requiring more than two satellites for circumpolar coverage), the experts in the field support the use of these systems in such applications. For example:
A current NASA paper (“The case for launching a meteorological imager in a Molniya orbit” by Lars Peter Riishojgaard, Global Modeling and Assimilation Office), asserts that the most effective way of providing a satellite system for meteorological monitoring at higher latitudes, is to use a Molniya system:
http://www.wmo.int/pages/prog/www/OSY/Meetings/ODRRGOS-7/Doc7-5 (1).pdf
A European Space Agency paper (“HEO for ATM; SATCOM for AIR TRAFFIC MANAGEMENT by HEO satellites”. Final Report, 2007) concludes that a Tundra orbit would take more satellites than Molniya, for coverage of northern latitudes for Air Traffic Management (ATM) applications; and
A presentation at International Communications, Navigations and Surveillance Conference, 2009, “SATCOM for ATM in High Latitudes”, Jan Erik Hakegard, Trond Bakken, Tor Andre Myrvoll, concludes that three satellites in Tundra orbit would be required for ATM at high latitudes. See: http://i-cns.org/media/2009/05/presentations/Session_K_Communications_FCS/01-Hakegard.pdf
There is therefore a need for an improved satellite system and methods for providing coverage of high latitudes, particularly for meteorological monitoring and communications applications.